1. Field of the Invention
The present invention relates to a semiconductor optical modulator that plays an important role in optical communication systems and optical information systems and more particularly, to an electroabsorption-type semiconductor optical modulator with a low chirping characteristic, which is preferably used for long-distance transmission of information.
2. Description of the Prior Art
In recent years, with the increasing transmission speed and length or distance of optical communication systems, a problem in the conventional direct modulation method of a semiconductor laser has been becoming conspicuous.
Specifically, in the direct modulation method, the "wavelength chirping" phenomenon that the wavelength of an output light beam from the laser fluctuates or deviates dependent upon time tends to occur on modulation of the beam. Thus, the wavelength of the modulated light beam tends to deviate from a specific oscillation wavelength.
When the modulated light beam is transmitted through an optical fiber, the waveform of the transmitted beam will be distorted or degraded due to the existence of the dispersion of the fiber. This distortion or degradation of the transmitted beam will be conspicuous more and more as the transmission distance increases.
Especially, this problem is serious for optical information systems using the conventional 1.3-.mu.m optical fibers with no dispersion. Even if the optical fibers of this sort and a semiconductor laser with a 1.55-.mu.m oscillation wavelength and a low propagation loss are used, the transmissible distance cannot be increased due to the above chirping phenomenon.
This problem can be solved by using the conventional external modulation method, where an external optical modulator is provided separately from a semiconductor laser, and a constant optical output from the semiconductor laser is modulated by the external modulator. Therefore, in recent years, a lot of the external modulators of this sort have been vigorously developed.
Within the external modulators of this sort, there are two types, one of which uses a dielectric material such as LiNbO.sub.3 and another of which uses a semiconductor material such as InP or GaAs.
Recently, the semiconductor optical modulator using the semiconductor material has been expected to be improved. This is because the modulator of this type is capable of not only integration with other optical components/elements such as optical amplifiers and electronic circuits/devices such as Field-Effect Transistors (FETS) but also miniaturization and operating-voltage reduction.
There are two types, the "electroabsorption" type and the "Mach-Zehnder" type, in the semiconductor optical modulators of this sort.
In the optical modulators of the "electroabsorption" type, a phenomenon that the optical absorption edge of a semiconductor material is shifted toward a long-wavelength side upon application of an electric field is utilized. There are the Franz-Keldysh effect for a bulk semiconductor material and the quantum-confined Stark effect for a multiple quantum-well (MQW) as the phenomenon of this sort.
In the optical modulators of the "Mach-Zehnder" type, a phenomenon that the refractive index of a semiconductor material is changed upon application of an electric field is utilized. There are known the electrooptic effect such as the Pockels effect for a bulk semiconductor material and the quantum-confined Stark effect for a Multiple Quantum-Well (MQW) as the phenomenon of this sort.
With the semiconductor optical modulators of the Mach-Zehnder type, the chirping phenomenon can be completely prevented from occurring based on their operating principle. However, the modulators have an interference-type waveguide structure (not a simple linear waveguide). Therefore, the modulators have a problem that not only the fabrication processes but also the driving or operating method are complicated.
On the other hand, with the semiconductor optical modulators of the electroabsorption type, although the chirping phenomenon is much lower than that in the direct modulation method of the semiconductor laser, the chirping phenomenon cannot be completely prevented.
Generally, when an electric-field corresponding to an electric signal is applied to a semiconductor optical absorption layer of a semiconductor optical modulator of the electroabsorption type, the absorption coefficient of the layer increases and at the same time, the refractive index of the absorption layer changes toward a positive side (i.e., increases). Consequently, the positive chirping occurs, resulting in degradation in dispersion resistance and limit in transmission distance.
Recently, however, it was confirmed by experiments that the chirping phenomenon could be suppressed if an electric-field corresponding to an electric signal is additionally applied to a semiconductor optical modulator of the electroabsorption type while a fixed bias electric-field is applied to the same modulator. This is termed the "prebias" method. This method is prospective because it will enable the ultra-high-speed and long-distance optical communication using the semiconductor optical modulators of this type.
An example of the conventional semiconductor optical modulators of the electroabsorption type is disclosed by K. Yamada et al., in the paper, 1995 IEICE General Meeting, pp. 349, C-349, published in 1995. This modulator has a bulk InGaAsP absorption layer with an absorption-edge wavelength or bandgap wavelength of 1.5 .mu.m. Incident light has a wavelength of 1.55 .mu.m. When an electric-field corresponding to an electric signal is additionally applied to the modulator while a fixed bias electric-field is applied to the same modulator, the dispersion resistance can be improved at a transmission speed of 10 Gb/s. This means that the limit in transmission distance can be overcome even at this transmission speed.
Another example of the conventional semiconductor optical modulators of the electroabsorption type is disclosed by K. Morito et al., in the paper, 1995 IEICE Electronics Society, pp. 301, C-301, published in 1995. This modulator is integrated with a Distributed FeedBack (DFB) semiconductor laser. This paper describes that when an electric-field corresponding to an electric signal is additionally applied to the modulator while a fixed bias voltage of 1.1 V is applied to the same modulator, the dispersion resistance can be improved. Thus, the optical transmission is successful at a transmission speed of 10 Gb/s over a transmission distance of 100 km.
With the above-described "prebias" method, such a configuration as shown in FIG. 1 is used. In FIG. 1, a bias-Tee circuit 305 is provided between a signal source 304 and a semiconductor optical modulator 302. The signal source 304 outputs an electric signal for modulation toward the modulator 302 through the bias-Tee circuit 305. The electric signal from the source 304 is additionally applied to the modulator 302 while an offset bias voltage is applied to the modulator 302 by a voltage source 306 in the circuit 305.
Incident light is emitted from a light source 303 to the modulator 302 through an optical fiber 307. The incident light is modulated by the applied electric signal to be transmitted to a receiver 301 located far away from the signal source 304 through an optical fiber 307.
With the configuration shown in FIG. 1, if the offset or bias voltage from the voltage source 306 is increased, the refractive index change of a semiconductor optical absorption layer of the modulator 302 on modulation is turned to be negative; i.e., the refractive index of the absorption layer decreases.
At this time, the change of the refractive index of the absorption layer will increase at the same offset/bias voltage, even if a detune, which is defined as the difference between the wavelength of the incident light and the absorption-edge (or, bandgap) wavelength of the absorption layer, is decreased.
However, the details about the behavior of the refractive index change with respect to the detune and the offset/bias have not been understood. Also, the optimum values of the detune and the offset/bias for suppressing the chirping phenomenon have not been understood. Accordingly, the practical stability of the conventional prebias method against the fluctuation in detune and/or prebias has not yet been made clear.
To solve this problem, it is necessary to clarify the mechanism of suppressing the chirping phenomenon and the stable chirp parameter against the detune fluctuation.